Not 100% sure, but it may be that in general, anal cancer is squamous cells and rectal cancer is adenocarcinoma. See https://pmc.ncbi.nlm.nih.gov/articles/PMC5224838/. For me I had a large tumor in the rectum and the initial colonoscopy indicated a small polyp in my anus as well. The histology indicated squamous cells. The oncologist had me retested because the treatment is very different and she wanted to make sure they did not get the sample from the polyp- the tumor and polyp were not adjacent and separate.